Fibers produced from thermoplastic polymers such as polyesters and polyamides have high dimensional stability and good mechanical properties. Accordingly, they are widely used for building interior decoration, vehicle interior decoration, and other industrial products, as well as apparel products. However, as fibers come into wider use, they are now required to meet varied characteristics requirements and, accordingly, different techniques have been proposed to provide fibers having special cross-sectional features to achieve sensitivity effects such as texture and bulkiness. In particular, the “ultrafineness” of fibers has a large effect on the characteristics of the fibers themselves and the characteristics of the fabrics produced therefrom. Therefore, these techniques represent the mainstream technology in terms of control of cross-sectional morphology of fibers.
If single component fiber spinning is applied to production of an ultrafine fiber, it will be impossible to obtain a fiber with a diameter smaller than about several micrometers even if spinning conditions are controlled with high accuracy. Thus, the multicomponent fiber spinning technique has been employed to convert a sea-island composite fiber into an ultrafine fiber. This technique is designed to first form a fiber with a cross section in which a plurality of island domains of a poorly soluble component disposed in a sea domain of a highly soluble component. Subsequently, the sea component is removed from the fiber or a fiber product formed therefrom to produce an ultrafine fiber composed of the island component. Currently, this sea-island spinning technique has been improved to produce ultrafine fibers (nanofibers) having a nano-level extreme fineness.
Fibers with a monofilament diameter of several hundreds of nanometers have unique features such as soft touch and texture that cannot be achieved in common fibers with diameters of several tens of micrometers or ultrafine fibers (micro-fibers) with diameters of several micrometers. Therefore, they can serve to produce such products as artificial leather and new tactile textiles, and they also serve to manufacture sports clothing that requires windproofness and water repellency, by taking advantage of their dense fiber structures. Nanofibers, furthermore, are able to get into very small grooves while increasing in specific surface area, and they can capture contaminants very efficiently in their extremely small interfiber gaps. With these characteristics, nanofibers have been used as industrial materials for wiping cloth and precision polishing cloth for precision equipment.
Having minimal fineness, as described above, these nanofibers can exhibit excellent quality. Nevertheless, they have some disadvantages such as poor mechanical characteristics including low resilience and bending strength. From the viewpoint of material mechanics, a simple decrease in fiber diameter causes a decrease in geometrical moment of inertia (material stiffness) in proportion to the fourth power of the fiber diameter. As a result, nanofibers by themselves have been useful for only limited applications as fiber products.
To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 2007-262610 proposes a technique for after-intermingling of a sea-island composite fiber that can form an ultrafine fiber (nanofiber) with an average fiber diameter of 50 to 1,500 nm and a general purpose fiber with a single fiber fineness of 1.0 to 8.0 dtex (about 2,700 to 9,600 nm).
It is true that the technique proposed in JP '610 seems to be able to provide fabric with improved mechanical characteristics because filaments with larger diameters will have major influence on the mechanical characteristics (for instance, resilience and bending strength) of fabrics produced therefrom.
In the technique proposed in JP '610, however, a fiber with a large diameter is used with a sea-island composite fiber to produce a combined filament yarn first, and then this combined filament yarn is interlaced, followed by carrying out sea removal treatment. This leads to a large unevenness in the distribution of nanofiber filaments in the cross-sectional direction or plane direction of the fabric. As a result, fabrics produced by the technique proposed by JP '610 are partially uneven in mechanical characteristics (such as resilience and bending strength) and water absorption capability. This is a disadvantage in applying the technique to manufacturing clothing. In the case of lining and other materials that come into direct contact with the skin, in particular, such a fabric can cause an uncomfortable sensation due to the peculiar texture of nanofiber. As a natural consequence, furthermore, such a fabric is also partially uneven in surface characteristics. This makes it very difficult to successfully apply such a fabric to high accuracy polishing material and wiping cloth that require high uniformity. This results from the temporal state where mutually independent sea-island composite fiber (groups of ultrafine filaments) and other fibers coexist in a pseudo-restraint condition in the fabric and, accordingly, it cannot be avoided as long as the after-intermingling technique is used.
To prevent an uneven distribution of ultrafine fibers caused by after-intermingling as described above, an effective method may be first forming a sea-island composite fiber having a cross section in which islands with large fiber diameters (island diameters) and those with small fiber diameters coexist and subsequently producing a fabric by interlacing this sea-island composite fiber, followed by removing the sea component, as proposed by Japanese Unexamined Patent Publication (Kokai) No. HEI 5-331711 and Japanese Unexamined Patent Publication (Kokai) No. HEI 7-118977.
JP '711 proposes a technique for composite fibers with uneven fineness having a cross section of a sea-island structure with a fineness of 1.8 denier (13,000 nm) or more in the outer portion and a fineness of 1 denier (10,000 nm) or less in the inner portions, with the fiber in the outer portion having a fineness three times or more that of the fiber in the inner portions.
Thus, the technique proposed in JP '711 provides products which, after removal of the sea component, contain fibers with large diameters in the outer portions and fibers with small diameters in the inner portions. The technique can produce a combined filament yarn with a cross section having a pseudo-porous structure. Capillarity of this porous structure serves to allow water on the surface of a combined filament yarn to move quickly. Fabrics produced from this combined filament yarn, therefore, can serve to provide a comfortable textile.
In the case of using the technique proposed in JP '711, however, water existing near the surface of the combined filament yarn is pulled into (absorbed by) the combined filament yarn. Accordingly, in a high temperature, high humidity atmosphere, moisture will be accumulated in the combined filament yarn although the humidity inside the clothes can be decreased temporarily in the initial period. Finally, the entirety of the cloth will become moist, resulting in an unpleasant sensation due to the moisture. In the case of using the technique proposed in JP '711, furthermore, fibers with a large diameter exist outside the cross section as described in Examples. As a result, prolonged treatment in a 5.0 wt % NaOH aqueous solution heated at 90° C. is necessary for complete sea removal, that is, removal (elution) of the sea component from the interior. Thus, the degradation of the remaining components cannot be ignorable. The technique proposed in JP '711 substantially makes use of fibers with large diameters (micro fibers or larger). Therefore, degradation of the remaining components is not taken into consideration. When using a nanofiber, however, it suffers an increase in specific surface area, leading to problems such as serious degradation of the remaining components, deterioration in mechanical characteristics, and coming-off of nanofiber filaments that will cause a reduction in overall quality.
For the technique given in JP '977, a proposal has been made concerning a composite yarn (combined filament yarn) composed of a polyamide fiber with a single fiber fineness of 0.3 to 10 denier (5,500 to 32,000 nm) in the core portion and a polyester fiber with a single fiber fineness of 0.5 denier (6,700 nm) or less in the sheath portion.
It is true that due to the use of a polyamide fiber as core component, the technique proposed in JP '977 is expected to serve to develop good mechanical characteristics such as preferred resilience and bending strength, as well as soft texture that is characteristic of the polyamide fiber.
The technique proposed in JP '977 substantially makes use of fibers with diameters larger than those of micro fibers. To make good use of the ductility of ultrafine fiber, therefore, it is necessary to adopt a polyamide fiber as core component and an ultrafine polyester fiber as sheath component. Accordingly, this will result in a difference in shrinkage between the core component and the sheath component, leading to bulkiness. On the other hand, as the core component having a large fiber diameter moves (shrinks) largely in the sheath component having a small fiber diameter, the technique proposed in JP '977 may also cause a variation in fabric characteristics due to an uneven distribution of ultrafine fiber filaments. Further, since the combined filament yarn is composed of different types of polymers, the compatibility between the core component and the sheath component (ultrafine fiber) is poor. Therefore, there is concern that deterioration in quality may be caused by a nap raised by friction.
Japanese Unexamined Patent Publication (Kokai) No. HEI 8-158144 examines a technique that uses a sea-island spinneret and proposes a technique relating to an spinneret designed to produce a sea-island composite fiber that contains island domains that have different cross sections (in terms of fiber diameter and cross-sectional fiber shape).
The technique proposed in JP '144 is an spinneret designed to feed a composite polymer flow containing sea component flows surrounded by an island component and unsurrounded island component flows to a confluence (compression) portion. Due to this effect, sea component flows that are not surrounded by the island component join adjacent island component flows into one island component flow. This phenomenon is caused to take place randomly to produce a combined filament yarn in which fiber threads with a large fineness and fiber threads with a small fineness coexists. To make this occur, JP '144 is characterized in not controlling the arrangement of the island domains and sea domains. The technique proposed in JP '144 is limited in controlling the fiber diameter although the size of the space between the split flows and feed holes serves to control the pressure and thereby control the rate of polymer discharge from the discharge holes. To form nano-level island domains by using the technique proposed in JP '144, the polymer feed rate per feed hole at least for the sea component should be as small as 10−2 g/min/hole to 10−3 g/min/hole. Accordingly, the pressure loss which is proportional to the polymer flow rate and wall distance and represents the major feature of JP '144, will be nearly zero, suggesting that the technique is not suitable for producing nanofibers with high accuracy. In fact, ultrafine yarns produced from the sea-island composite fibers obtained in the Examples have a fineness of about 0.07 to 0.08 d (about 2,700 nm), suggesting that they cannot serve to produce a nanofiber.
Thus, there have been strong expectations for the development of a sea-island composite fiber suitable for producing, with high quality stability and post-processability, fabrics that have good mechanical characteristics required of fabrics such as resilience and bending strength while maintaining functions (texture, function, etc.) characteristic of nanofibers after the removal of the sea component.